Method of concentrating isotopic carbon



July 1, 1952 M. CALVIN ETAL METHOD oF CONCENTRATING IsoToPIc CARBON IAII S\m mtv INVENTORS. MELI//N CALVI/V JOHN W. WE/GL M M M M une N3 2G NS di Patented July 1, 1952 UNITED STATI-:s l'wiTElxiTorrlcisfl Y Melvin Calvin and John W. Weigl, Berkeley, Calif., assignors to the United States of Amerf ica as represented by the United States Atomic` I t `Energy Commission Applicationncemher 21,1949,saria1N9.13i,27o

This invention relates to a new chemical method for the concentration of the isotopes of carbon and more particularly it relates to a method of separating the light and heavy isotopes of carbon contained in a mixture thereof by the preferential absorption of the lighter isotopes by living chlorophyllic organisms.

, vRecently there; has developed a great need for large. amounts of concentrated isotopes of variousf chemical elements including carbon. These isotopes are of enormous lvalue toy research Workers inthe elds of tracer chemistry and medicine. It is to be noted that the concentrated isotopes of .carbon are of great utility, particularly in the elds of organic chemistry and the biological sciences. 'Reactions may be studied in greater detail and structures which were heretofore uncertain may now be positively established by means of these isotopes. For these reasons it is to be expected that'large quantities of the isotopes of carbon will be the needs in these elds.

In the past, various methods for the concenltration of isotopeshave been devised and used'. Perhaps the rst Yof these was the mass-'spectro-v graphic method which separated only minute quantities of isotopes. Other methods for eiect-v and thermal diffusion. Other methods of enrich.

ment include electro-chemical means, fractional distillation, and chemical exchange reactions. All these methods are valuable for the concentration of particular isotopes and find applications in isotopic production at the present time. However, many are too costly in operation/for the concentration of carbon isotopesl or are not feasible for other reasons.

Now it has been found that the heavier isotopes of carbon may be separated from the lighter isotopes present in a mixture by feeding carbon dioxide formedfrom the mixed isotopes to chlorophyllic organisms whereupon the carbon dioxide containing the lighter isotopes is preferentially absorbed and utilized bythe organisms. The principal reaction involved in the process appears to be a photosynthesis reaction in which the simple carbon dioxide is converted into complex` molecules incorporated inthe live ing tissue ofthe organism In this manner it is possible to separatethe heavierl carbon isotopes 13 and lli4 from the lighter carbon isotope 12 contained in a, mixture. (Hereinafter said isotopes will beV abbreviated as 012,013 and C14.)

Y `11 claims. (01.204-157) It is therefore an object of the present invention to provide a useful, practicalphotosynthesis method for the concentrationof C13 andother demand .t to satisfy exposed to heavier isotopes of carbon.

It is a further object of the present invention to provide a method'for' the concentration ofthe C13 by a process using photosynthesis reactions.

It is a further object of the present invention to provide a photosynthesis method forthe concentration of the heavy isotopes of carbon which is readily adaptable to commercialproduction of said isotopes. 'j j j tj Y It is a further object of the present invention to provide a method for thel enrichment ofC13 in mixtures of gases using low cost materialsin easily operated processes and equipment. i

Other objects and advantages will' be apparent from the following` descripton taken in conjunction with the accompanying drawing of a lloW diagram illustrating the invention;

vThe basic photosynthesis reactions which occur in green plants are not 'fully understood. However, it is known that plantsY which contain chlorophyll are able to vutilize carbon dioxide to form complex organic molecules when they are sunlight'. Accordingly, present theories indicate that light' furnishes the energy required for the reduction of thejcarbon dioxide in such reactions. t Manytheories have suggested mechanisms for the photosynthesis reduction of carbon dioxide by chlorophyllic organisms but the present invention is not to be bound by the validity of any such explanations but only by the fact that the concentration'of Aisotopes takes place as herein described. If normal carbon dioxide which contains about 1% of 'C13 is reacted under controlled conditions with plant life, the 'C12 is preferentially removedffromvthe carbon dioxide feedstock. Therefore, :the residual carbon dioxide is'proportio'nately riclieri'nCll..v

It will be appreciated,"o'f course, that theheavier isotopes also are anabolized and, therefore, that organisms" nourished on y carbon vdioxide which has been enriched in the heavierA isotopes will themselves be proportionately richer in the heavier isotopes than .the original carbon dioxide feedstock and it might be saidthat such organisms were enriched with respect to said isotope. The organisms nourished with the original feed stock carbon dioxide will become relatively :de-v pleted with respect to heavier iso'tope'contt and, therefore. should. Abe c'liscarded from'f'the system. In the event that the organisms at any stage of the process .becomey enriched with'the desired isotope, carbon dioxide formed by'ltiurx'i-l may be vregardedas acmplex equilibrium state f betweerfthe `quantities'of the heavier isotopes in thefresidual carbon dioxide and the heavier car-` the tissues of the organism. Ult follows `that the' 4 nitrogen, in a concentration not exceeding about percent or not exceeding the limit which is normally tolerated by the organisms. The carbon dioxide is introduced at such a rate and in such afashion that a preclet errnined` proportion of the carbon.v dioxide `is"l absorbed {by the algae. Under the described conditions about 100 grams per minute of carbon dioxide which normally bon isotopes present in the compounds formingv enrichment of the residual carbon dioxide may contains about 1% of C1302 is introduced. Thev reactionis allowed to proceed at such a ratev that about 80% of the incoming carbon dioxide reacts. VUnder' these vconditions the eliiuent 20% of the .carbon dioxide isfound to contain about ploying appropriate batchwisev cyclic or counter- 1.038 Q1 3Q2. ySince the algae become relatively impoverished with respect `to C13 and therefore the einciencyof the process tends to drop after an extended*periodi"of operation, the algae are current cyclic processes with the reintroduction of enriched material into the sequence ofoperations, any desired degree of enrichment can be effected. 'If the feedstock'mixture'employed in the'pr'ocess contains the radioactive isotope C14 it ris likewise cor`icentre'.`ted. It has been found that, by employing organisms which are able to derive all of :the carbonrequired in their metabolic processes from carbon dioxide', the full benefits and advantages of the process may be obtained v,The algae have been quite suitable for the purposes of the invention. Other growing organisms, i. e., organisms capable of undergoing photosynthesis, may also be Vused with appropriate modification of the process.

.'.In"gen,eral', the process may be'operated as a batch process by employing aclosed system which is adaptd'tofthe growing of the organism employed. Appropriate apparatus may be equipped forte'rnperature control, for agitating and illuminating the mixture, .for introducing the carbon dioxide feedstock and for collecting and removing the reactedcarbn dioxide. An appropriate nutrient medium, suchas Hoaglands solution, inayalso be supplied'inany appropriate fashion. WhenA the process is operated in Va continuous 1nu1t is tage. fashion, provision'must b made for burning of the organisms in which the proportion of 'the carbonisotope has been increased and for the reintroduction of the isotopically enriched carbon dioxide formed thereby.

f Particular details of the process will become more apparent from aconsideration of the following examples illustrating the operation of typical processes in accordance. with the present invention.. vThe specific values indicatedY in said examples are ffor illustrative purposes only and arenot to be,- construed as limitativeof the invention y yEavampleA-w "The rst stage of the procesa'V as illustrated in the drawing, canjbeioperated in a single stage batchwise fashion forenriching normal carbon dioxide with respect to C1? as follows: 75 kilograms of algae (Chlorella pyrenoidosa or sceneclesmfu's) which have been centrifuged to Vremove excess water.y areqintroduced into the c'lc'psedj system 'together V4with an appropriate quantity of a nutrient 'solution, preferably with low' intensity illumination. t The operation'of the processis begun by illuminating the algae with visible llightA from a `suitablesource and by introducing carbon dioxide." Sunlight vhas been found entirelysuitable for illuminating the algae while artiiiciallight may also be employed. The carbon dioxide is preferably introduced in admixture with a diluentgas, such as ordinary air or removed and discarded and new algae are introduced. The relative quantities of carbon dioxide presentin the various gaseous mixtures may be continuously indicatediby means of infrar. red adsorption cell equipmentjand the isotopic composition of the variousK admixtures of carbon dioxide are determined byV massA spectrographic analysis."

ExampleJ3` t The operation o f the complete-process, as illustrated in the drawingpwill vbe v'described for ef-A fecting an enrichment of v'normal carbon dioxide with respect to C13. Each of the stages 'of the two stageA process illustrated in the drawing operates in a fashion very similar to the batchwise operation of the vfirst` stage, as described in Example A.A However, in thepresent case appropriate modification is made for the burning of algae derived from the second stage and introduction 'of the' carbon dioxide so derived into. the first stage. Accordingly, a recycling effect is obtained and the process-is operated in a continuous counter-current fashion. It will be ap-f preciated that the secondstage is of a proportionately Vsmallercapacity `since the quantity: of carbon dioxide which must tbe treated is less than that `'of' the rststage. j

With the apparatus arrangedV to provide for continuous Acounter-current operation, as shown in the drawing, the following sequence of events occurs: With about kilograms of centrifuged algae (Chlorella lpyrenoidosa.; or sceneclesmtcsy present in the first stage ofthel systemand about 15 kilograms of algae inthe second stage ofthe system, normal carbon' dioxide `is introduced in the first' stagegofl the system,` as described"in` Example A, and theA eiuentresidual 20% of the carbon dioxide is introduced intoY the se cond stage. As a steady state isapproached in each stage, algae are withdrawn Afrom thefsecondfsage and burnedl to form carbon dioxide at the rate of aboutk 19.1 g./ I n., whichcarbon dioxide is f ed into the first stage.' The efiluentcarbon dioxide from the' first stage comprisescarbon dioxide which has beensingly enriched 'withespect to C13 and the effluent carbon dioxide from the second stage comprises`carbon dioxideiwhich has been doubly enrichedfwith respect to C13. @Under the steady state conditions described,` vit isfound that the singly enrichedcarbon dioxide contains about 1.038% of C1302 and the doubly enriched carbon dioxide contains labout 1.078%161302 Vas compared to the originalcarbonl dioxide which contains about 1% of C1302. fOn th'ebasisof Va g./m. input of normal carbondioxide to the first stagev and with-recycling, about 23.9- g./m.

of singly enriched carbon dioxide and 4.8 g./`m.

of doubly enrichedcarbon dioxide are produced in the respective stages. For a two stage process without recycling, it has been found that only about 4.0 g./m. of a similar product is obtained. In the process employing recycling, the basis for the calculation of the 80% carbon dioxide utilization, of course, takes into consideration the quantity of carbon dioxide being recycled.

The foregoingiexamples illustrate typical processes operated in accordance with the invention. It will be apparent that the process resembles a multiple plate distillation process in which each stage is similar in operation to the plates of the distillation system. In the present case each stage (plate) achieves an increase of 4% over the original quantity of C13 present. By supplying an appropriate number of stages any degree of enrichment can be effected. While the invention has been described with respect to an enrichment with respect to C13, it is applicable to the separation of any of the heavier carbon isotopes from the lighter isotopes present in a mixture. Thus, C11 present in a mixture may also be recovered, in which-case, the quantity of C11 present is easily determined by means` of a thin mica window Geiger-Mller tube radioactivity counter.

O-ther organisms besides those indicated may be employed in the process, the requirements for such organisms are believed apparent from the foregoing description. Under some conditions of operation, barley seedlings have proven satisfactory. The temperature conditions under which the various organisms thrive vary somewhat but, in general, a temperature of between about 15 and 35 C. is satisfactory. The'concentration of C02 maintained over the algae in each stage should be between about 1/2 and 10% by volume of the gas which in contact with the algae mixture and with the remainder consisting of a gas which is inert with respect to the photosynthesis reaction.

The carbon dioxide which has been suiiiciently enriched with the desired isotope can be ccnverted into barium carbonate by absorption in a barium salt solution to provide an easily handled article of manufacture.

As employed in the present application, the term chlorophyllic is intended to indicate that the organism is capable of utilizing carbon dioxide in a photosynthesis reaction.

While the salient features of this invention have been described in detail with respect to two embodiments, it will be apparent that numerous modiiications may be made within the spirit and scope of the invention, and it is intended to cover all such that fall Within the scope of the appended claims.

What we claim is:

1. In a method of concentrating an isotope of carbon including the enrichment of carbon dioxide with respect to a heavy isotope thereof, the steps comprising continuously contacting a gaseous mixture containing an inert gas and 0.5 to 10 volume percent of carbon dioxide which contains a mixture of heavy and light carbon isotopes with chlorophyllic plant life in a iirst closed system to preferentially remove carbon dioxide containing the lighter isotopes to yield carbon dioxide singly enriched with respect to the heavier isotopes, continuously removing said enriched carbon dioxide from the system, continuously contacting said singly enriched carbon dioxide with chlorophyllic plant life in a second closed system to yield carbon dioxide doubly enriched with respect to said heavy isotope.' continuously removing said doubly enriched carbon dioxide'from the system, removing quantities of plant life from said second system, burning said removed plant life to form carbon dioxide, and introducing said formed carbon dioxide into said iirst system.

2. In a photosynthesis method for `concentrating an isotope of carbon including the enrichment of carbon dioxide with respect to a heavy isotope thereof.'` the steps comprising contacting chlorophyllic plant life in a first closed system with normal carbon dioxide contained to the extent of about 0.5 to 10 volume percent in admixture with an inert gas whereby the C1202 is preferentially removed by the plant life and carbon dioxide enriched with C1302 is produced therein, separating the enriched carbon dioxide from the system, contacting said enriched carbon dioxide contained to the extent of about 0.5 to 10- volume percent in admixture with an inert gas with chlorophyllic plant life in a second closed system whereby the carbon dioxide isdoubly enriched with respect to C1302, and recovering said doubly enriched carbon dioxide from the system.

' 3. A method for concentrating C13 in normal carbon dioxide comprising contacting said carbon dioxide present to the extent of about 0.5 to

-l0 volume percent in admixture with an inert gas with plant life in a closed system whereby said plant life preferentially absorbs C1202 from the carbon dioxide in said mixture to produce carbon dioxide singly enriched with C1302, separating said enriched carbon dioxide from the system, similarly contacting said enriched carbon dioxide with a second mass of plant life whereby the growing plant mass preferentially absorbs C1202 from the carbon dioxide kproducing carbon dioxide doubly enriched with C1302, separating the doublyenriched carbon dioxide from 'the system, and burning quantities of the second mass of plant life to produce carbon dioxide, recycling said produced carbon dioxide including the repitition of said steps to achieve further enrichment.

4. A photosynthesis method for concentrating heavy isotopes of carbon contained in normal carbon dioxide comprising contacting said carbon dioxide while present to the extent of 0.5 to 10 volume percent in admixture with an inert gas with green plant life in a closed system to preferentially absorb C1202 contained in said carbon dioxide and produce carbon dioxide enriched with C1302, and separating said enriched carbon dioxide from the system.

5. A process of enriching carbon dioxide with respect to the heavy isotopes of carbon contained therein comprising contacting said carbon dioxide while present to the extent of 0.5 to 10 volume percent in admixture with an inert gas with a rst mass of chlorophyllic organisms in a closed system to preferentially absorb lighter isotopic carbon dioxide in a photosynthesis reaction resulting in an enrichment of the residual carbon dioxide with respect to the heavier isotopes of carbon, separating said enriched residual carbon dioxide from said organisms, repeating said steps of contacting and separating said carbon dioxide as a multiple sequence of stages including said operations of contacting and separating employing multiple masses of organisms, producing carbon dioxide from quantities of said masses of organisms, introducing said produced carbon dioxide into earlier stages in said sequence, and recovering carbon dioxide enriched with respect amaai?,-

7 to Ysaid heavy. isotopes `from .said sequence o off stages. 'i f j" u i,

' 6; The. Yprocess as in c claim 5, wherein vsaid organismscomprise algae. v

:7. The process as -in claim 5, wherein quantities of the organisms derived from said first stage also are burned and the carbon dioxide so produced' is introduced.v into said first stage.

v8. Theprocess of enriching carbon. dioxide with respect to the heavy isotopes, of carbon contained therein comprising contactingsaid carbon diox-` ide While present tothe extent of 0.5 to 10 volume percent inI adrnixtureV with an vinert gas with chlorophyllic plant life 'in a closed system.

9: Inv amethod for concentrating C1302 contained in normal carbon dioxide, the step come prising contacting said carbon dioxide While present to the extent .of 0.5-'tio 10. volume percent in admixture with an inert gas with algae in a closed system until the'volume thereof has been reduced .toless than about 20% of the original volume, .and thenseparating sa-id carbon dioxide 7from the system.. a

'10. In a method of concentrating an isotope oi carbon including the enrichment of carbon dioxide With respect to the. heavy isotopes thereof,

the steps comprising continuously contacting a gaseous mixture containing an inert gas and 0.5 to 10 volume percent of carbon dioxide which contains a mixture of heavy and light carbon isotopes With algae in a iirst closed system until about 80% Vof theoriginal volume thereof is absorbed and the residual carbon dioxide is singly,

dioxide .fromv the system, continuously removing quantities of algae from said second system, burning said removed algae to form carbon ldioxide, and introducing said formed carbon dioxide into said first system. l

11. A method for concentrating heavy isotopes of carbon from carbon dioxide containing a mixture of heavy Aand light isotopes thereof comprising contacting said carbon dioxide While presentto the extent of about 0.5 to 10 volume percent in admixture with an inert gas With chloroy phyllic plant life in a closed system whereby said plant life preferentiallyv absorbs the lighter isobon dioxide, and recycling the carbon dioxideV formed thusly including a similar repetition of the foregoing steps, thereby effectinga further enrichment of said enriched carbon dioxide with respect to said heavier carbon isotopes. MELVIN CALVIN.

JOHN-W. WEIGL.

REFERENCES CITED The following references are of record in the le of this patent:y

. Rabinovvitch, fPhotosynthesisf? v01. 1^, (1945) Pollard et al., Applied Nuclear Physics, 2n ed., (1951), 13.214. 

8. THE PROCESS OF ENRICHING CARBON DIOXIDE WITH RESPECT TO THE HEAVY ISOTOPES OF CARBON CONTAINED THEREIN COMPRISING CONTACTING SAID CARBON DIOXIDE WHILE PRESENT TO THE EXTENT OF 0.5 TO 10 VOLUME PERCENT IN ADMIXTURE WITH AN INERT GAS WITH CHLOROPHYLIC PLANT LIFE IN A CLOSED SYSTEM. 